A federal official overstepped his authority by allowing three states to demand proof of citizenship on the national “motor-voter” forms that help many Americans register to vote, the Obama administration and allied groups argued Wednesday in a case that pits one part of the federal government against another.

The League of Women Voters said eligible voters in Kansas, Alabama and Georgia will be turned away in a pivotal election year because the U.S. Election Assistance Commission’s executive director waded into a “clear question of policy” that can be tackled only by commissioners of the independent agency.

“The practice is clear,” Michael C. Keats, an attorney for the league, told U.S. District Court Judge Richard J. Leon in the District of Columbia, pleading with him to block changes to the federal form’s instructions in all three states until their lawsuit is decided on the merits.